Some of Africa's oldest and biggest baobob trees have died

The trees -- located in Zimbabwe, Namibia, South Africa, Botswana and Zambia -- have died completely, or partly, according to the study. Some of them dated back to the times of the ancient Greeks.

Researchers found that nine of the oldest 13 baobab trees and five of the six biggest ones have partially or completely perished in the past 12 years.

The baobab tree, often referred to as the "tree of life" for its ability to produce nutrient-rich fruit even during Africa's harsh dry season, can live to be 3,000 years old, and can grow as wide as the length of a bus. In the past, they have been used as a prison, a barn and a bus shelter, according to the website of Kruger National Park in South Africa.

When they've lost their leaves, the trees appear to be upside-down, with their branches resembling roots that extend upward. Given their age and their resilience to the often-harsh weather conditions in Africa, they have been the source of local stories and legends.

"It is definitely shocking and dramatic to experience during our lifetime the demise of so many trees with millennial ages," Adrian Patrut of Babeș-Bolyai University in Romania, one of the study's co-authors, told AFP.

The Panke tree, which was oldest of the dying trees, lived for 2,500 years until its stems collapsed in 2010-2011, according to the study.

It's still unclear what is driving the baobab deaths. The authors believe that climate change is the culprit.

"We suspect that the demise of monumental baobabs may be associated at least in part with significant modifications of climate conditions that affect southern Africa in particular. However, further research is necessary to support or refute this supposition," the authors wrote.